Getting the Job
You don't. You can't apply for the job in the museum until you have taken the Underground Tour. Once you finish that, you can ask about the job.
In the Museum at Night
At night, you can slip past the guard in the Security Room when her shadow is not seen.
(see related questions)
You must go on the Underground Tour before you can apply for the job at the museum.
At the museum, you can't enter until you have taken the Underground Tunnel Tour. (see related question)
The red-haired guard you meet at the Museum is at the Clown Store after the painting is stolen. The female guard is in the Security Room. Slip past her at night to enter the museum. The next day, get the timecard from the guy guard (at the Clown Store) and take it to the Security Room in the museum.
You likely mean the security guard at the Museum on Counterfeit Island. (The only detective is the police inspector.) You cannot enter the museum and try to get a job there until you have completed the underground tour. (see related question)
drink bleach, everyone knows that...
when you enter the island, keep going left, past the museum. then click on the sign that says "city docks".
Just run past the window when the guard's shadow isn't visible on the shades.
To pass the Security Room, wait until the guard's shadow is no longer on the window.
first u have to talk to a guy on the dock. then u have to apply for the museum job. :)
Slip out the door to the left into the main gallery and go to the painting (The Scream).
Once you have completed the Underground Tour, you can apply for the job at the museum. Once past the security guard, you can wander the museum. However, you are better off completing the painting-swapping task, and then the Forgery Training, before touring the museum. (For one thing, you will be getting a key to the Supply Room when you finish training.)
There are no snowballs on Counterfeit Island. (see the related question)